This invention relates to low profile magnetic components, and more particularly relates to such components including planar magnetic winding structures, such as inductors and transformers, in which the windings are composed of stacks of interconnected layers of conductor patterns.
The trend toward miniaturization in power electronics requires high power density and low profile magnetic components, which in most cases are the bulkiest circuit components. Reducing the size of such components requires higher drive frequencies, in order to obtain the same performance as a larger magnetic component. All of these factors of smaller size, higher power density and higher operating frequency lead to considerably increased heat generation. As the size of such components decreases, the available surface area through which the heat can be guided towards the environment decreases rapidly, while the total losses remain about the same.
Planar winding structures consist of a stack of layers each containing part of the total winding structure, an insulating layer used to prevent electrical contact between the turns in adjacent layers, usually consisting of a flexible, non-conducting, low permittivity, high temperature resistant polymer, and a contacting structure that permits electrical contact between turns in adjacent layers where needed. The winding structures are optimized with respect to winding losses, and are usually made by etching or stamping or sometimes by folding. Contacts are usually made by soldering or via plating.
The use of such magnetic components is always accompanied by dissapative losses in the core and windings. Such losses decrease the efficiency of the magnetic component and the electronic circuit of which it is a part, and increase the temperature of the component and the surrounding area, changing the electrical characteristics and reducing the lifetime of the component and other components in thermal contact with it.
In general, winding losses are due to the interaction of the winding current with a local magnetic field, due largely to leakage flux from the windings and to stray fields near the gaps in the core. The leakage flux is largely determined by the dimensions of the winding structure and is more or less evenly distributed over the winding window. However, the stray fields near the gap are local in nature and can give rise to large local increases in winding losses. In addition, such local losses will give rise to local thermal runaway phenomena, so called xe2x80x9chot spotsxe2x80x9d.
In planar winding structures, air gaps which induce fringing fields can form a significant portion of the core window height. High frequency magnetic field components in the winding current then induce large eddy currents in the conductors near the air gaps, and as a result, current crowds toward the edges of these conductors. The eddy currents are most severe for the conductors adjacent to the gaps because the fringing fields are strongest in these regions, and therefore it is preferred to locate the windings as far away from the gaps as possible.
The one-dimensional analytical loss equations popularly used for calculating high frequency losses in ungapped transformer designs do not adequately predict the winding losses of a gapped structure due to the presence of these air gaps.
It is difficult to minimize these gap-related losses using traditional wire wound construction techniques, since it is difficult to achieve the desired separation between air gaps and windings in a reproducible way.
Increasing the spacing between the windings and the air gaps inevitably decreases the cross-sectional area available for the winding in a fixed core geometry. This reduction in winding area increases the DC resistance of the coil, and depending on the balance of low-frequency and high-frequency currents in the core, the benefits of reducing high-frequency losses by avoiding the keep-away regions may be offset by increased DC losses.
Accordingly, it is an object of the invention to provide a low profile magnetic component with a planar magnetic winding structure, having reduced winding losses.
It is another object of the invention to provide such a component containing air gaps in the core, having reduced winding losses.
According to the invention, there is provided a planar magnetic winding structure comprising a core of two or more core components having mutually facing planar surfaces separated by at least one air gap having a height g, and a stack of winding layers, each layer including one or more turns, in which structure the edges of the windings are separated from the air gap by a distance of at least 2g, and preferably by a distance of at least 3g.
According to one embodiment of the invention, the core of the planar magnetic winding structure comprises a first lower core component having a planar portion and two or more spaced-apart upstanding portions having planar upper surfaces, the upstanding portions defining a space to accommodate the stack of winding layers, the core also comprising. a second upper core component having a planar lower surface, the planar upper surfaces of the upstanding portions of the first core component and the planar lower surface of the second core component defining the air gap g.
Preferably, the stack of winding layers in the space between the upstanding portions of the lower core component comprises two or more sub-stacks, each sub-stack having the same number of turns in each layer, the sub-stacks proximal to the air gap g having a smaller number of windings per layer than the sub-stacks distal from the air gap g.
Most preferably, the number of turns per layer in each successive sub-stack from the most distal sub-stack to the most proximal sub-stack is smaller than the number of turns per layer in the preceding sub-stack.
A planar magnetic winding structure in accordance with the invention, such as a transformer or inductor, having an air gap in the core with a xe2x80x9ckeep awayxe2x80x9d region of 2 to 3 times the gap height in which there are no windings, reduces high frequency winding losses by 35 percent or more, without appreciable increases in low frequency winding losses. Such structures are useful, for example, in electronic ballasts for the lighting industry.
FIGS. 1a, 1b and 1c are an expanded perspective view, a top view and a side view, respectively, of a schematic representation of a planar transformer construction of the prior art;
FIGS. 2a, 2b and 2c are side section views of a schematic representation of a planar transformer construction of the type shown in FIG. 1, having a winding configuration of the prior art, a stepped winding configuration of the invention, and a tapered winding configuration of the invention, respectively;
FIG. 3 is a graph of planar inductor winding resistivity in ohms versus frequency in Hz for the planar transformer constructions of FIGS. 2a through 2c; and
FIGS. 4 is a bar graph of relative inductor winding losses in percent versus frequency in kHz for the planar transformer constructions of. FIGS. 2a and 2c.